The present invention relates generally to UV lamps used for treating photopolymerizable films, and specifically to microwave-powered lamps where the microwave cavity is independent of the optical system.
UV radiation is used to photochemically polymerize (cure) relatively thin films on various surfaces. The established technology for performing the polymerization generally comprises either an electrode or microwave-powered ultraviolet lamp, as disclosed, for example, in U.S. Pat. No. 4,042,850. The electrode or microwave power is dissipated in a plasma-filled bulb. The component elements of the plasma are chosen primarily to radiate light at some desirable wavelength or range(s) of wavelengths. In general, the plasma-filled bulb is situated in an optical system that has the desired effect of focusing the UV light in a manner that improves the efficiency of a given process.
In the case of a typical microwave-powered lamp, one or more magnetrons are used to generate microwave power, which is then fed into a microwave cavity containing the plasma-filled bulb. The microwave cavity serves the dual purpose of containing substantially all the microwave energy and focusing the UV light output from the bulb. Thus, if a new optical system is desired, the properties of the microwave cavity must also change. Typically, designing a new microwave cavity that also meets the new optical requirements is a highly cumbersome task and, in practice, it is more common to alter the polymerization process rather than altering the optical/microwave system.
Typical microwave-powered UV lamps operate in a regime of very high power densities, where several hundred watts of microwave power may be absorbed by the plasma in a relatively small volume. Due to inherent inefficiencies in the plasma, some of the microwave power is converted to heat and dissipated in the walls of the bulb, a phenomenon known as xe2x80x9cwall loadingxe2x80x9d. Wall loading imposes the restriction that, in typical operation, the plasma-filled bulb must be cooled by some external means to prevent overheating and promote long bulb life. Normally, this is accomplished by circulating air or some other coolant over the surfaces of the bulb. The operable power density of a given plasma-filled bulb is limited by the surface area of the bulb and the available practical means for removing heat from that surface.
In some instances, it is desirable to run a photopolymerization or other light sensitive process in an environment other than air. Such instances can include those where the light sensitive process is also undesirably chemically sensitive to one or more of the gaseous elements that are present in air, such as oxygen. Another instance can be where the optimum wavelength of light for a given process may not be readily transmitted through air. This portion of the light spectrum is usually referred to as xe2x80x9cvacuum UVxe2x80x9d. Instances such as these are often referred to as the xe2x80x9cinerted processesxe2x80x9d due to the required presence of some inert gas or vacuum between the light source and the process.
It is an object of the present invention to provide microwave-powered lamp where the microwave cavity is separate from the optical system to allow for rapid adaption of the lamp to any reasonable optical system.
It is another object of the present invention to provide a microwave-powered lamp that constricts the plasma toward the center of the bulb, thereby reducing the temperature of the bulb envelope to allow operation at much higher power densities.
It is still another object of the present invention to provide a microwave-powered lamp where the cooling system for the bulb is separate from the curing atmosphere, thereby allowing for application in an inerted or vacuum UV process.
In summary, the present invention provides a microwave-powered lamp, comprising a microwave source; a microwave cavity operably coupled to said microwave source, the microwave cavity being substantially cylindrical about a centerline; an elongated bulb disposed along the centerline of the microwave cavity; and a reflector operably associated with the bulb to direct radiation generated by said bulb to a product being cured. The bulb may be enclosed by a solid barrier such that cooling gas used for cooling the bulb is isolated from the curing environment. The microwave cavity is separate from the function of focusing the radiation output from the bulb so that changes to the optical system can be made without also modifying the microwave cavity.
These and other objects of the present invention will become apparent from the following detailed description.